Technické zdroje

Intergranular Corrosion Tests for CRA’S

headerThere are a number of recognised tests when it comes to the intergranular corrosion testing of corrosion resistant alloys (CRA). Each has similarities and differences (both in the test solution, the test sample and the pass/fail criteria), which it is a good idea to familiarise yourself with.

This document describes all three of these tests for four common standards, and describes the differences. Just remember that these tests were devised for specific groups of alloys and should not be applied to all CRAs!  This document also points out which alloys the tests are intended for, and recommends the most appropriate test when none is specified.

Why were these tests designed?

welderIntergranular corrosion tests tests are primarily designed to detect carbide precipitates on grain boundaries. However, although some tests also claim to detect intermetallic phases, inter granular corrosion tests should not be relied on for this, as the response of CRAs to the test solutions vary widely.

These tests were introduced when the carbon content in CRAs typically hit 0.08% maximum, improving weldability with lower carbon contents, where this can be difficult to achieve.  Nowadays, AOD (argon oxygen decarburisation) melting means that most CRAs have a 0.03% carbon maximum, and are usually 0.01 to 0.02% carbon.  This means that carbide formation during welding is highly unlikely, and modern CRAs should easily pass one of these tests.  However, the increased sourcing of CRAs from the Far East means that carbon content may actually be higher, regardless of what it says on the certificate.  For this reason, intergranular corrosion tests are becoming more important.

1. The Tests

Table 1 (below) shows the various methods for four common standards: ASTM A262, ASTM G28, ISO 3651-2 and SEP 1877.

The latter is a German test, but it is often called up in European projects.  ASTM A262 method A is only a screening test and is not included here, while method D was deleted many years ago.

  • The ASTM A262 method B and ASTM G28 method A tests are the same, except for their duration.  ASTM G28 lists alloys and specifies the test exposure for each.  It is mostly 120 hours, but some alloys (such as C-276 (UNS N10276) have a test exposure of 24 hours, because of the high corrosion rates found in this solution.  These tests are similar to ISO 3651 method C and SEP 1877 method 2, except for the shorter duration and the lower acid content.
  • ASTM A262 method C (Huey test) is the same as that specified in ISO 3651-1 (not listed here).  This test should be avoided for most CRAs, except in the circumstances described below, in section 4.
  • ASTM A262 method E is sometimes called the Strauss test, and is the same as ISO 3651-2 method A, except for the duration.  As 15 hours is a difficult test period in practice, and is only a minimum, conducting the test for 20 hours overnight means that the two tests are equivalent.
  • ASTM A262 method F is a more aggressive version of method E, as are ISO 3651-2 method B and SEP 1877 method 1.  None of these are identical and cross substitution of results is not possible.
  • ASTM G28 method B is a more aggressive test than method A and is intended for more corrosion resistant alloys, as listed in the document.
  • SEP 1877 method 3 is a special environment only suitable for the Ni-Mo alloys, as described below, in section 4.

2. Pass/Fail Criteria

The pass/fail criteria in these tests needs some thought.  Where it is assessed purely on corrosion rate, the pass/fail criterion should be sought from the customer, unless it is written in their purchase specification.  Where the criteria is for internal assessment only, then it will be necessary to test material judged to pass in the same test, as a comparison.  The exception to this is ASTM G28 method A, where rates for the onset of intergranular corrosion are described in reference 1.

steel with water dropsFor many alloys, the onset is at 0.46mm/y, and a common pass/fail criterion would be 0.7 or 0.8mm/y.  Some suggested criteria for the listed alloys are shown in Table 2, below.  Good material should have a corrosion rate substantially less than the threshold value.

For alloys not listed in this standard, it will be necessary to carry out some tests on material in various conditions to determine the threshold.

Some suggested pass/fail criteria for highly corrosion resistant alloys tested in the ASTM G28 method B are shown below, in table 3.

Where a bend test is required, the sample must be bent over a mandrel of a diameter related to the specimen diameter, without any cracking, as determined by inspection under a low power microscope and a microsection in any suspicious areas.  The specifications for bend testing are shown in Table 4.  All bend tests require a tensile test machine or similar, with suitable grips and sufficient strength to bend the strongest alloy.

3. Test Samples

Samples may be rectangular or cylindrical to suit the test vessel, but where a bend test is required, it is suggested that the sample be a minimum of 100mm long.  The most convenient size is often a cylinder 10mm diameter by 100mm long, but other dimensions are possible to suit particular product forms.

Some standards require the test sample to be aged prior to testing, to simulate the precipitation that might occur during welding.  ASTM A262 suggests ageing all 300 series alloys at 650 to 675°C for one hour.  It does not say what to do about duplex or super austenitic alloys.  ASTM G28 does not require samples to be heat treated.

ISO 3651-2 is specifically for duplex and all austenitic stainless steels, and recommends ageing at two different temperatures:

  • 700°C for 30 minutes.
  • 650°C for 10 minutes

Both of these tests should be followed by water quenching.  Where one of these is not specified, the 700°C test is the default.  Alternatively, a welded test-piece in the as-welded condition may be tested.

SEP 1877 does not require ageing, although it might be required in a specification calling up this standard.

4. Selection of a Test Method

The test methods in Table 1 were designed with specific alloys in mind, but the proliferation of new CRAs has meant that the selection of a suitable test method is difficult.

ASTM A262 was designed for austenitic stainless steels, specifically the 300 series alloys.  The Huey test (method C) is only for molybdenum-free grades, such as 304L, as alloys with molybdenum show high rates of attack in this test.  Hence, it is normally only used for stainless steels intended for nuclear service, or another application involving nitric acid.  It has been used for the duplex stainless steel 329 (UNS S32900), which has been used for nitric acid cooler tubes.

Methods B and E are suggested for all grades of 300 series austenitic alloys, while method F is suggested for cast 316-type alloys.

ASTM G28 was designed for nickel-based alloys, and a list of applicable alloys for methods A and B is in the standard.  For an alloy that is not listed, look for an alloy with a similar composition to a listed alloy to determine the most suitable test.

ISO-3651 is the most rigorous in terms of defining test methods, and says which alloys should use which test methods as follows:

METHOD A

Austenitic alloys where Cr>16% and Mo≤ 3%           (e.g. 304L; 316L)

Duplex alloys where Cr>16% and Mo≤ 3%                (e.g. 2101; 2304; 2003)

METHOD B

Austenitic alloys where Cr>20% and 2% 4%    (e.g. 317L)

Duplex alloys where Cr>20% and Mo> 2%                (e.g. 2205; Z100; 2507)

METHOD C

Austenitic alloys where Cr>17% and Mo> 3%           (e.g. 6% Mo; 904L)

Austenitic alloys where Cr>25% and Mo>2%            (e.g. alloy 28)

Duplex alloys where Cr>20% and Mo> 3%                (e.g. 2205; Z100; 2507)

What happens to dual-category alloys?

It can be seen that some alloys fall into two categories, and no guidelines are given for selecting the most appropriate.

SEP 1877 also gives guidance on which of its methods apply to which alloys:

ISO 3651-2 method A/ ASTM A262 method E             316L, 2205, 904L, alloy 800

METHOD 1                                                                               alloy 329, superduplex

METHOD 2                                                                               nickel alloys (C-276 etc)

METHOD 3                                                                               Ni-Mo alloys (B-2, B-3, B-4)

Where a standard is included in a purchase specification, this must be followed, although it is advised that unsuitable tests, such as the Huey test for duplex and super austenitic stainless steels, are queried. More suitable alternatives are offered in Table 5, below.

Where no specific test is specified, Table 5 suggests the most appropriate alongside a second choice, if the first is declined.

5. Conducting the Tests

When a test has been selected, follow the detailed directions in that standard for making up the test solution, the preparation of test samples and the finish to be used.  There are other requirements in these standards over details that are not discussed in this document, and these should be followed, such as conducting bend tests.

 REFERENCE

 1. R Baboian, Corrosion Tests and Standards: Application and Interpretation, ASTM Manual Series MNL 20, second edition, 2005, ASTM, PA, USA.

TABLE 1: The test solutions and exposures for some common intergranular corrosion tests.

TEST

METHOD

SOLUTION*

DURATION

(hours)

PASS/FAIL

ASTM A262

B

50% sulphuric acid

25g/L ferric sulphate

120

Corr. rate

C

Huey Test

65% nitric acid

5 x 24

Corr. rate

for last 24h

E

16% sulphuric acid

100g/L copper sulphate

copper turnings

15 min.

Bend test

no cracks

F

50% sulphuric acid

120g/L copper sulphate

copper turnings

120

Corr. rate

ASTM G28

A

50% sulphuric acid

25g/L ferric sulphate

24 or 120

Corr. rate

B

23% sulphuric acid

1.2% hydrochloric acid

1% ferric chloride

1% cupric chloride

24

Corr. rate

ISO 3651-2

A

16% sulphuric acid

100g/L copper sulphate

copper turnings

20

Bend test

no cracks

B

35% sulphuric acid

110g/L copper sulphate

copper turnings

20

Bend test

No cracks

C

40% sulphuric acid

25g/L ferric sulphate

20

Bend test

No cracks

SEP 1877

1

34% sulphuric acid

110g/L copper sulphate

copper turnings

15

Bend test +

Corr. rate

2

40% sulphuric acid

25g/L ferric sulphate

24

Bend test +

Corr. rate

3

10% hydrochloric acid

24

Bend test +

Corr. rate

* All solutions are boiling

TABLE 2: Suggested pass/fail criteria in ASTM G28 method A test, based on data from ref. 1.

ALLOY

UNS No.

TEST DURATION (h)

THRESHOLD

 

(mm/y)

PASS/FAIL

 

(mm/y)

Alloy 20

N08020

24

0.61

1.0

Alloy 800

N08800

120

0.46

0.8

Alloy 825

N08825

120

0.46

0.8

Alloy 600

N06600

24

0.61

1.0

Alloy 625

N06625

120

0.46

0.8

Alloy C-276

N10276

24

6.35

9.14

      Alloy C-22

N06022

24

0.80

1.20

Alloy 59

N06200

24

0.60

1.00

Alloy C2000

N06059

24

0.70

1.00

TABLE 3: Suggested pass/fail criteria in ASTM G28 method B, based on data from Haynes International and VDM Metals.

ALLOYUNS No.TEST DURATION (h)THRESHOLD

 

(mm/y)

PASS/FAIL

 

(mm/y)

   Alloy C-276   N10276       24       0.80    1.50
     Alloy C-22   N06022       24       0.10    0.25
     Alloy 59   N06200       24       0.08    0.15
   Alloy C2000   N06059       24       0.07    0.15

TABLE 4:  Bend test criteria for several standards.

STANDARD

BEND ANGLE

(°)

MANDREL DIA.

 

ASTM A262 E

180

Thickness/diameter of sample

ISO 3651-2

90 – 180

2 x thickness/diameter of sample

SEP 1877

DIN 50914

DIN 50914

TABLE 5: Suggested test methods for different corrosion resistant alloys.

ALLOY

UNS No.

FIRST CHOICE

SECOND CHOICE

316L

S31603

ISO 3651-2 Method A

ASTM A262

Method E

310

S31000

ISO 3651-2 Method A

ASTM A262

Method E

904L

N08904

ISO 3651-2 Method C

ASTM A262

Method B

6%Mo

N08367

ISO 3651-2 Method C

ASTM A262

Method B

2101

S32101

ISO 3651-2 Method A

ASTM A262

Method E

2003

S32003

ISO 3651-2 Method A

ASTM A262

Method E

2205

S32205

ISO 3651-2 Method A

ASTM A262

Method E

ZERON 100

S32760

ISO 3651-2 Method B

ISO 3651-2

Method C

20

N08020

ASTM G28

Method A

ISO 3651-2

Method C

825

N08825

ASTM G28

Method A

ISO 3651-2

Method C

625

N06625

ASTM G28

Method A

ISO 3651-2

Method C

C-276

N10276

ASTM G28

Method A

ISO 3651-2

Method C

Alt SEP 1877 Method II

C-22

N06022

ASTM G28

Method B

ASTM G28

Method A

Alt SEP 1877 Method II

B-2; B-3

N10665/N10675

SEP 1877

Method 3

*

* No second choice recommended